IMO, Boardwalk and BC/YC are among the most convenient because you can easily walk or boat to Epcot and DHS. You would still need to bus to MK and AK, but I find these resorts are pretty centrally located so the bus rides to the other two parks are still manageable.
Poly has the monorail to MK, and you can walk to TTC to get on the monorail to Epcot. Contemporary has a walking path to MK, and you can ride the monorail to TTC where you can transfer to the Epcot monorail. So these two are about tied for second most convenient, IMO.
AKL will be the closest resort to AK, but you'd still have to take a bus. No alternate transportation there.